Generally, a composite body is produced from a fibrous substrate the interstitial cavities of which are filled with a coating material. In the case of a carbon-carbon composite body, for example, a substrate of carbon fibres may be impregnated with (a) a resin which is then polymerised and pyrolysed, or (b) with a pitch which is then pyrolysed. It is also possible to deposit pyrolytic carbon within the interstitial cavities of such a substrate from the vapour phase.
To assist the coating material to perform its filling function, it is necessary for the interstitial cavities of the substrate to be uniform in size and to be uniformly distributed, and to be in good communication with one another and with the exterior.
An object of this invention is to provide such substrates.
In addition, for certain purposes, attempts have been made to obtain composites having substantially isotropic physical properties. For this purpose, it is necessary for the fibres of the substrate to be orientated in a plurality of non-coplanar directions about a given point.
A further object of the invention is to provide substrates having such characteristics.
Finally, for certain purposes, it is desirable to produce composite materials having maximum density. To achieve this object, it is necessary for the ratio between the volume of the substrate and the volume of the final composite body to be as high as possible.
Accordingly, a further object of the invention is to provide substrates having a high ratio of volume of substate to the volume of the final composite.
To achieve these objects, a number of methods have already been proposed. One of the simplest of these consists in uniting together groups of filaments into basic shaped members such as rods and then assembling these members along three mutually orthogonal axes. Two principal difficulties have been encountered in putting this method into practice. The interstitial voids are large because of the method of construction, the voids constituting approximately 40% in the case when cylindrical rods are used. In effect, when cylindrical filaments are aligned contiguously in three mutually orthogonal directions, an elementary cube having an edge which measures half the pitch of the assemblage contains 3.times.1/4 cylinders of fibres and thus the ratio of the volume of the substrate to the total volume is approximately 60%. In addition, it is difficult for a coating material to gain access to the interstitial voids, in such a structure, particularly when such coating material is deposited from the vapour phase.
It is difficult to envisage mechanising a process of constructing the substrate rob by rod when the pitch is fine.
Another known method consists in threading cloth made from threads which have been woven in two mutually orthogonal directions onto metal wires perpendicularly to the wires, and in subsequently replacing the wires by strands of fibres. This method has the following disadvantages. Only a limted number of the cloths can be threaded onto the metal wires, thus restricting the thickness of the final substrate; the method is difficult to put into practice since the positioning of the cloths is a delicate operation and the replacement of the metal wires by the threads of fibre is a lengthy and laborious technique: finally, the method cannot be automated when the distance between two metal wires becomes small, for example, of the order of one millimeter.